A Roberval-type load cell uses a Roberval mechanism which is not affected by a position of objects, and is widely used for weighing in weighers (scales). FIG. 4 is a schematic view of a general Roberval-type load cell 41. As shown in FIG. 4A, the general Roberval-type load cell 41 includes an upper beam section 42 located at an upper portion, a lower beam section 43 located at a lower portion, a fastening section 44 fastened to a stationary object such as a device body, and a movable section 45 applied with a vertical external force, and entirely has a rectangular shape. Four strain gauges 46 are attached to both ends of an outer peripheral surface of the upper beam section 42 and both ends of an outer peripheral surface of the lower beam section 43.
When a load is applied to the load cell 41 in a direction indicated by an arrow in FIG. 4B, the entire load cell 41 strains. According to this strain, the strain gauges 46 attached to a left upper portion and a right lower portion extend, and the strain gauges 46 attached to a right upper portion and a left lower portion contract, among the strain gauges 46 attached to the four locations in FIG. 4B. As the load applied to the load cell 41 increases, a degree of strain of the strain gauges 46 increases, as a matter of course. The strain gauge 46 has a characteristic in which its electric resistance changes according to a degree of extension or contraction. Because of the characteristic, the electric resistance of the strain gauge 46 is converted into a voltage by a Wheatstone bridge circuit, and a voltage value is obtained, thereby detecting a magnitude of the load applied to the load cell 41.
However, since the Roberval-type load cell 41 has the above configuration, the load cell 41 strains significantly and thereby the strain gauges 46 may possibly be damaged if an excess load is applied to the load cell 41. To prevent an excess load from being applied to the load cell 41, generally, a stopper member 47 is provided outside of the load cell 41 or on a body of the load cell 41. For example, as shown in FIG. 4, the stopper member 47 is provided below the movable section 45. In a layout in which the stopper member 47 is provided with a space below the movable section 45, as shown in FIG. 4, when the movable section 45 is displaced by a distance equal to the space and contacts the stopper member 47, the movable section 45 is not displaced any more. This makes it possible to prevent an excess load from being applied to the load cell 41.
FIG. 5 shows another mechanism. FIG. 5 is a perspective view of a Roberval-type load cell 51 having a mechanism different from that of FIG. 4. In the load cell 51 of FIG. 5, a fastening section 54 is provided with a groove at a near side in FIG. 5, and a plate-shaped stopper member 57 is fastened to the groove. In addition, a movable section 55 is provided with a groove at a near side in FIG. 5. A width of the groove formed in the movable section 55 is greater than a width of the stopper member 57. A vertical slight space (gap) is formed between the groove of the movable section 55 and the stopper member 57. In the load cell 51 of FIG. 5, having this configuration, when the movable section 55 is applied with a load and displaced, inner side of the groove formed in the movable section 55 contacts the stopper member 57, so that the movable section 55 is not displaced any more. This makes it possible to prevent an excess load from being applied to the load cell 51.
Patent Literature 1 discloses another mechanism (see FIGS. 1 to 4 in Patent Literature 1). Patent Literature 1 discloses that stoppers are provided from both of an upper beam and a lower beam toward inside. These stoppers are provided such that there is a little space (gap) in a rightward and leftward direction. Because of this, when the load cell is applied with an external force and thereby strains, these stoppers are displaced in the rightward and leftward direction and contact the load cell, so that the movable section is not displaced any more. This makes it possible to prevent an excess load from being applied to the load cell.